The invention relates to a method and a device for detecting a watermark embedded in a signal in which method a plurality of frames of the signal is combined to a detection set for one detection event.
The invention also relates to an apparatus for recording and/or playback of a signal and to a system for monitoring a broadcast signal.
Watermarks are imperceptible or hardly perceptible marks embedded in a signal such as audio, still pictures, animations, video, etc. They are embedded in the signal in such a way that they are difficult to remove. A watermark may comprise additional information such as, for example, information about the source or the copyright status of documents or audiovisual programs. Watermarks allow tracing of piracy and support the protection of intellectual property.
A known method of detecting a watermark comprises a correlation step for correlating the signal with locally stored watermarks. The result of such a correlation is subsequently evaluated. When the correlation of the signal with a given watermark is, for example, larger than a given threshold value, this watermark may be considered to be the watermark embedded in the signal. A correlation with many different watermarks may also be performed, with the watermark having the highest correlation being considered as the watermark that has been used.
Since the signal may be subjected to various signal operations such as MPEG compression, DA/AD conversion, NTSC/PAL conversion, as well as cropping and scaling, a detection based on single frames is often unreliable in the sense that it leads to many missed detections. The article “On the Reliability of Detecting Electronic Watermarks in Digital Images” by A. A. C. Kalker, G. F. C. Depovere and J. P. M. Linnartz, published in the Proceedings of Eusipco 1998 (Rhodes, Greece) gives a plausible explanation for the fact that the combination of a plurality of frames, for example n, to a detection set enhances the reliability of the detection process by a factor of √n. However, this applies only to n uncorrelated frames. It is common practice to combine consecutive frames. However, these are generally highly correlated.